The invention is bases on a priority application EP 02 360 069.5 which is hereby incorporated by reference.
The invention relates to optical fiber transmission systems.
As is known, in fiber optic lines transmitting optical signals carrying information, Polarization Dependant Loss (PDL) namely quantifies the passive elements sensitivity to the state of polarization of the carrier signal. PDL, normally expressed in dB, has a given value for each passive element of the line and can be defined as 10 log(Tmax/Tmin) where T is the optical transmittance (or power) taken over the entire polarization-state space.
PDL is an undesirable phenomenon since it causes loss varying with the state of polarization of the carrier signal. The impact of PDL on network performance is increased signal distortion and consequently higher bit-error-rate (BER). Besides, PDL effect varies as a function of the wavelength of the carrier signal. In addition, as the state of polarization of the carrier signal evolves randomly with time, PDL effect leads to time-varying network performance.
PDL of passive elements is usually characterized as a localized phenomenon and depends namely on the technology and design of the elements. By way of example, elements like filters, multiplexers, demultiplexers or isolators lead to on negligible PDL at the end of a transmission line. Moreover, PDL can also occur for example at the interface between elements. Only some specific optical fibers (OF) are designed to have low PDL.
Furthermore, aging can significantly increase the PDL of the line Thus, in present lines, existing solutions do not allow to compensate for all kinds of PDL, as well as PDL occurring or varying in time.
An aspect of the present invention is to implement an optical fiber transmission system having at any time a very low PDL effect, at least for carrier signal(s) with wavelength(s) in a given spectral band included in the transmission band.
For this purpose, the invention provides an optical fiber transmission system for transmitting at least a first carrier signal with a wavelength in a first spectral band. The system includes an optical fiber, at least a first Raman amplifier module for launching into said optical fiber a first pumping signal at an appropriate wavelength for a stimulated Raman amplification of said first carrier signal. The first Raman amplifier module includes first pumping means with two optical sources. One optical source emits at the appropriate wavelength, a linear beam along a first axis of polarization, the other optical source emits, at the appropriate wavelength, a linear beam along a second axis of polarization distinct from said first axis.
The module further comprises first combination means arranged to combine the beams in order to provide said first pumping signal.
The first Raman amplifier module includes first polarization tuning means arranged to superimpose in real time at least one of the axes of polarization to a Polarization Dependant Loss (PDL) axis.
Raman amplification is classically used in ultra long transmission system for compensating attenuation occurring along the travel distance while lowering the amplified spontaneous noise. It is known that, in a Raman amplification, gain is varying with the state of polarization of the carrier signals, depending on the relative polarisation state between the carrier signal and the pumping signal. This phenomenon is called Polarization Dependent Gain (PDG).
The first Raman amplifier module of the invention enables to induce a controlled level of Raman PDG as a function of time. The Raman PDG substantially compensates the PDL occurring in the system before the Raman module that is on the emitter side, since the invention makes it possible to superimpose the first and/or the second axis of polarization of the first pumping signal to axis or axes xe2x80x9cresponsiblexe2x80x9d of PDL. Thus, an appropriate Raman amplification as a function of the state of polarization of the first carrier signal is provided.
By way of example, such Raman PDG can compensate the PDL of multiplexers, optical isolators and filters included in an Erbium Doped Fiber Amplifier (EDFA) disposed before such Raman module.
Besides, the first pumping signal of the invention only amplifies first carrier signals in a certain spectral band corresponding to the first spectral band of the invention. Thus, other carrier signals in second spectral band(s) included in the transmission band are not amplified. Moreover, amplified first carrier signals, located very near to the limits of the first spectral band, are less amplified by the first Raman module. Such non-amplified or less amplified carrier signals still suffer from the PDL effect. Two of the spectral bands can be chosen so as to include less amplified first carrier signal(s) to provide them a better amplification.
Advantageously, the system of the invention can further comprise at least a second Raman amplifier module for launching into the optical fiber a second pumping signal at an appropriate wavelength for a stimulated Raman amplification of a second carrier signal. The second Raman amplifier module is distinct from the first Raman amplifier module, and includes a second pumping means, a second combination means and a second polarization tuning means similar to the first polarization tuning means.
In this way, each spectral band of the transmission band can be associated to a dedicated Raman amplifier module used to correct PDL. Thus, the invention counteracts the variations of PDL with the wavelength.
In an embodiment of the invention, each of the polarization tuning means comprises power variation means arranged to vary the optical power of at least one of the optical sources.
PDL corrections are improved by properly adjusting the amplitude of at least one linear polarization.
With the configuration of the pumping means of the invention, the resulting polarization of each pumping signal can be:
linear, for example if one of the sources is off,
circular, for example if optical sources provide beams at a same level of optic power and with axes of polarization perpendicular to each other,
or more broadly elliptic.
By way of example, one optical source can provide a beam with a variable power (from zero to a given maximal power) thanks to power variation means, while the other source provides a beam with a constant power (zero or a given value).
Another example is to have both optical sources providing beam with variable powers thanks to power variation means, with the second axis of polarization perpendicular to the first axis.
In all configurations of the invention, it is recommended to have means keeping the total optical power of the pumping signal constant in order to keep the optical output power of carrier signals constant.
Preferably, each source can be chosen among power adjustable laser diode and a power adjustable fiber laser.
Power adjustment of such sources is performed through electrical voltage or current variations.
In an embodiment of the invention, each of the polarization tuning means comprises rotation means arranged to rotate in real time at least one of the axes of polarization.
In this way, for example, the first axis of polarization and/or the second axis of polarization can be rotated, independently or together, providing PDG counter-acting PDL.
Preferably, each of the rotation means of the invention can comprise at least a tunable Pockels cell tuned by a variable voltage.
The variable voltage induces birefringence in the Pockels cell, leading to axes rotation.
Two Pockels cells can be located near the optical sources before the combination of the linear beams or one Pockels cell can be located after the combination of the linear beams in the opposite side of optical sources. Advantageously, the system of the invention can further comprise feedback means controlling in real time each of said polarization tuning means.
Such feedback means make a diagnostic of the PDL corrections to apply for each group of carrier signals and thus command each of the polarization tuning means to adjust properly the amplitude and the direction of at least one of the linear polarization of beams.
To this end, the feedback means of the invention can comprise:
extraction means arranged to pick up a fraction of at least the first carrier signal,
selection means arranged to select at least a representative extracted carrier signal per spectral band,
analysis means arranged to analyze the sum of PDL effect and PDG effect on said representative extracted carrier signal,
command means arranged to master in real time each of the polarization tuning means.
The analysis means give access to the global effect of PDL and PDG, so that the latter is adjusted to cancel the former (PDL+PDG=0).
The representative carrier signal can be for example, the signal having the central wavelength in its spectral band and even a group of signals.
With the increased rate of information transmitted, an optical fiber transmission system is generally design to transmit DWDM (Dense Wavelength Division Multiplexing) carrier signals in a very large transmission band.
As carrier signals can be multiplexed, preferably the feedback means of the invention can comprise separation means arranged to separate the multiplexed extracted carrier signals.
Besides, the choice of analysis means depends on the state of polarization of the carrier signals, which can be scrambled signals, having adjacent carrier signals with a same or a crossed polarization.
The analysis means of the invention can be chosen from one or more from the following means: photodiodes, power spectrum analyzers and DOP measuring means.
A transmission system of the invention can comprise low frequency polarization scrambling means. Therefore, when the sum PDL+PDG differs from zero, there is a temporal modulation of carrier signals at the frequency of the scrambling (typically below 100 MHz). As the period of the scrambling is higher than the response time of photodiodes (xcx9c10 ns), photodiodes can be used to detect time variations of each representative signal. Thus, until these variations become substantially equal to zero, command means of the invention master each of the polarization tuning means.
A transmission system of the invention can transmit adjacent carrier signals having crossed polarization. When the sum PDL+PDG is different from zero, adjacent crossed signals do not have the same level of optical power. Therefore, power spectrum analyzers of the invention give information on the global effect of PDL and PDG.
The degree of polarization (DOP) of a carrier signal shows to what extent this signal is polarized. A transmission system of the invention can comprise low or high frequency polarization scrambling means and/or transmit adjacent carrier signals having crossed polarization. Therefore, when the sum PDL+PDG is different from zero, the DOP of carrier signal has a non-zero value. DOP measuring means of the invention can be used to detect this DOP variation for each representative signal. Thus, until this variation becomes substantially equal to zero, command means of the invention master each of the polarization tuning means.
Similarly, when adjacent carrier signals have parallel polarizations, DOP measuring means can be used to detect the DOP variation for each representative signal, initially equal to 1. Thus, until this variation becomes substantially equal to zero, command means of the invention master each polarization tuning means.
More broadly, an optical fiber transmission system can include several group of Raman amplifier modules associated with feedback means, for example repeated periodically in the fiber system in order to correct the accumulated PDL when needed.